Sunday, November 22, 2015

Switched power for the car

I own a Ford vehicle, and apparently Ford doesn't believe that 12V power should be switched with the ignition. This makes it inconvenient to power dashcams and GPS units as easily as you can with many other types of vehicles.

I really didn't want to start cutting into wires on my new car and I couldn't find any points in the under-dash fusebox to tap that had ACC switched power, so I built a circuit to sense the voltage and activate a relay when it was higher than the resting voltage of the battery.  When you start the engine, the voltage will go up to > 13 volts, when you turn it off, it should go down to < 12.5 or so.

I started with a circuit found online intended to cut off power when the input voltage dropped too low.  Here's the circuit (with final values in it)

I breadboarded it up and tested it by attaching it to my variable bench power supply and running the voltage up and down to make it trigger. I originally tried a 12V zener, but it triggered a bit too high, because the transistor has a saturation voltage of around 1 volt which needs to be added to the zener voltage to get the trigger voltage.  An 11 volt zener turned out to be about perfect. The original circuit also used a few hundred μF capacitor, but that took tens of seconds to trigger both ways, so I changed to a 2.2μF 680μF capacitor. (NOTE changed later to 680μF - my car drops voltage to < 12.6V occasionally when the battery is topped off, so in field testing the output actually switched off and on frequently - the larger capacitor keeps it running for a few minutes until the voltage comes back up again)

I also tested its current draw.  When in the off mode, this circuit draws less than my meter can measure, so it's less than 0.1 microamps. This is completely irrelevant to a car battery and won't run anything dead.  When in the on mode it draws about 40 milliamps to drive the relay, but the car is running then and that's a perfectly acceptable draw.

Once I was happy with the trigger voltage and speed, I took it out to the garage to test that it turned on and off as expected:

Once satisfied, I wired up a permanent version on a variboard.  Most of the parts are stuff I just keep around for throwing together random ideas.  The 12v relay was salvaged from my old lightning destroyed garage door opener before trashing it (I got 4 relays from that).

I designed and 3D printed a box to put it in complete with some lugs on the side to zip-tie it up under my dashboard.  The box design is up on YouMagine at this link.

I clipped the female socket off of a 12V Y adapter (the cheapest way to get one in the shop) and wired it to the ground and output, and used a standard fuse tap to get power. I'll find a ground under the dash somewhere and wire that to the ground, and lash the whole thing up there with zip ties.

Here's the box ready to go into the car:


Darryl Baker said...

An elegant solution. I'm just curious about the need. My cursory search of the internet seems to to say that Ford provides a few extra fuses in the stock fuse block that are powered and unused. That seems to suggest that the side of the fuse away from the alternator is available for connectivity. How I wouldn't be sure until I had a good look at the car. So I can't say whether cutting of wires would be necessary. So as I said you do have an elegant solution.

John Ridley said...

I spent some time under the dash with a multimeter and was unable to find any fuses or empty positions that were unpowered when the key was off. It seems like these days everything is continuously powered, and some communications goes on internally for the controller to soft power devices up and down.

If there are any switched lines, they must switch on a certain time in the off position rather than just key off.

It's pretty irritating that there's no documentation on this stuff. I suppose I'd have to buy the service manual for the car, which is probably several hundred dollars. I'm not that interested.

I didn't have to cut any wires here, I just used a fuse tap and a wire wrapped around a frame mounting screw for ground.